This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. This question and its answers are frozen and cannot be changed. More info: help center.

86 Answers
86

The PuTTY executables and source code are distributed under the MIT licence, which is similar in effect to the BSD licence. (This licence is Open Source certified and complies with the Debian Free Software Guidelines.)

Synergy lets you easily share a single mouse and keyboard between multiple computers with different operating systems without special hardware. It's intended for users with multiple computers on their desk since each system uses its own display.

UnxUtils: This is a port of various gnu shell utilities based on msvcrt.dll so it understands native windows paths - i.e. you don't need to map to a /cygdrive path. This is a key advantage over Cygwin if you have to interact with native windows commands or homebrew CL utilities.

Strings: is a very good way to scrounge through files for items of text. Many, many uses.

Flex: Really designed for writing lexical analysers, with a little bodge artistry and a C compiler it can be used as an uber-grep. I don't use it all that often but it can come in surprisingly handy in that role.

Fetchmail and Procmail: Core of my email system for well over a decade, since I had dial-up internet connectivity. If it ain't broke ...

rdesktop: an open source RDP (terminal services) client that works surprisingly well.

PythonWin:, particularly as packaged in Activestate Python. Python on Windows works a lot better than you might think. When used with COM Makepy it's really good for scripting COM APIs.

Leafnode: if you still read any of the newsgroups that still have decent active traffic this is quite a good way to do it. Again, a bit of legacy from my dialup days but it still gets used on occasion.

Abiword and Gnumeric: full featured wordprocessing and spreadsheet software that's far leaner and meaner than OpenOffice.

Xfig: Visio type diagramming tool with an odd user interface. Once you get used to the paradigm it's much easier on my poor old mouse hand than a modern direct maniulation interface. Worth a mention for the ergonomics.

Tcl/Tk: Overshadowed by Perl and Python, Tcl is very easy to embed C code into - it was designed specifically for embedding. Surprisingly useful nonetheless, and the Tk toolkit is very easy to whip up a GUI with. Modern versions support theming so your applications no longer have to look like Motif.

Ghostscript: One of the great unsung heroes of the open-source world. A free postscript interpreter with a whole ecosystem of derived items - PS and PDF viewers, PDF creation tools, printer RIPs and all sorts of Postscript conversion tools. Perhaps most widely used outside open-source circles (if not actively credited) in its role in the back-end of PDFCreator

That's just a sampling of the obscure stuff without mentioning Vim, LaTeX, Firefox, python, gcc, gtk & qt and the Berkeley TCP stack - to name but a few.

Linuxconf = Linuxconf comes with Mandrake Linux and Red Hat Linux, but is also available for most modern Linux distributions. You've probably encountered this tool before if you use one of these distributions, either as the whole package or in one of its modular components. Multiple interfaces for Linuxconf have been available for years, but now we're up to four: GUI, Web, command-line and ncurses.

Webmin = Webmin comes with, and was recently acquired by, Caldera Linux. This tool is not only available for most modern Linux distributions, it also runs on most major flavors of UNIX and is available in around twenty languages (though some modules are not available in all of the languages). As you might guess, Webmin is purely a web-based application and a heavily modular one at that.

KeePass is a free open source password manager, which helps you to manage your passwords in a secure way. You can put all your passwords in one database, which is locked with one master key and/or a key file.

I use many that I couldn't work without but that I don't consider "not very famous" (openssh, openvnp, apache, rsync, ...). Two very useful little utilities that many may not have heard of sprint to mind:

dstat - imagine vmstat, iostat, top, ps, as well as apache, mysql, etc. all able to output metrics on the same line at the same interval. cross-referencing app-level metrics with system-level metrics is huge.

I have been finding that many people don't yet know about Process Hacker. It's on par with Sysinternals' Process Explorer.

Edit in response to Greg's comment:
Sorry for the delay in responding... It also has 2 tabs that show services and TCP/UDP connection info which I think is really nice. You can get the same info in the services tab in Process Eplorer when sorted by tree view, but then you lose the ability sort within the services list.